This invention relates generally to borehole logging apparatus and methods for performing radiation based measurements. More particularly, this invention relates to a new and improved apparatus for effecting neutron porosity logging in real time wherein the improved nuclear logging apparatus comprises a measurement-while-drilling (MWD) tool.
Oil well logging has been known for many years and provides an oil and gas well driller with information about the particular earth formation being drilled. In conventional oil well logging, after a well has been drilled, a probe known as a sonde is lowered into the borehole and used to determine some characteristic of the formations which the well has traversed. The probe is typically a hermetically sealed steel cylinder which hangs at the end of a long cable which gives mechanical support to the sonde and provides power to the instrumentation inside the sonde. The cable (which is attached to some sort of mobile laboratory at the surface) is also the means by which information is sent up to the surface. It thus becomes possible to measure some parameter of the earth's formations as a function of depth, that is, while the sonde is being pulled uphole. Such measurements are normally done in real time (however, these measurements are taken long after the actual drilling has taken place).
A sonde usually contains some type of source (nuclear, acoustic, or electrical) which transmits energy into the formation as well as a suitable receiver for detecting the same energy returning from the formation. The present invention relates to logging apparatus wherein the source emits nuclear energy, and more particularly neutrons. When using this type of source, the source sends out "fast" (high energy) neutrons into the formation. The fast neutrons leaving the source enter the formation and slow down by losing energy as a result of collisions with the nuclei of the formation, finally becoming thermalized. By thermalized, it is meant that, on the average, the neutrons lose as much energy as they gain as a result of collisions, that is, they are in thermal equilibrium with the nuclei of the formation. After some time spent diffusing as thermal neutrons, they may be captured by one of the formation nuclei resulting in the emission of a gamma ray. The energy of the gamma ray emitted is characteristic of the particular nucleus involved. It is in this context that the term "thermal capture gamma ray spectra" is used. Examples of well logging tools of this type are disclosed in U.S. Pat. Nos. 3,379,882, 3,662,179, 4,122,338, 4,223,218, 4,224,516, 4,267,447, 4,292,518, 4,326,129 and 4,721,853.
Fast neutrons as a probe source are useful for several reasons. For example, chemical sources for the fast neutrons such as Am.sup.241 Be and Pu.sup.238 Be are readily available. Fast neutrons also have a reasonable degree of penetration into matter, and finally most importantly, neutrons can be especially useful for the detection of hydrogen. To understand the effect of hydrogen, it is helpful to use the analogy of a group of billiard balls in which the neutron and the hydrogen nucleus are balls having essentially the same mass while the nuclei of other elements in the formation are balls with much larger masses. Thus, if a neutron collides with the nucleus of an element other than hydrogen, it will generally lose very little energy. If it collides with a hydrogen nucleus, because the masses are nearly equal, it can lose all of its energy. The ability of a formation to slow down fast neutrons to thermal energy then depends primarily on the hydrogen density.
With regard to hydrogen density in a formation, two diametrically opposed situations may be considered. In the first situation, a group of fast neutrons leave a source and slow down in a formation free of hydrogen, and in a second situation, a group of fast neutrons leave a source and slow down in a formation which has a great deal of hydrogen in it. One expects and will find that the neutrons will have gone much farther away from the source in the first case than in the second case. As a result of the foregoing, a technique which has been in use in "wireline oil well logging" for more than thirty years is the measurement of the spatial distribution of slowed down neutrons. This technique is usually described as neutron porosity logging because the porosity of the formation is inferred from the measurement. Here it is tacitly assumed that the pores of the formation are filled with either water or oil (an assumption not always true since there may be gas or a mix of all three components). It is also assumed that the hydrogen density for oil and water are equal (that assumption is also not strictly true, but can be safely assumed for all practical purposes).
In order to construct a neutron porosity sonde which looks at the spatial distribution of slowed down neutrons, one needs a source of sufficient intensity (for example, 10.sup.7 neutrons/sec), and a detector separated from the source (for example, 15 inches). There needs further to be sufficient shielding between the source and detector to keep the radiation coming directly through the sonde to a minimum. Further features needed in the sonde involve reducing the response of the sonde to factors other than porosity, such as borehole size, salinity, etc. Evolution in the prior art of this type of sonde has consisted primarily in changes in the type of detector used. Originally, Geiger counters with heavy walls were used. These counters did not detect neutrons but rather gamma rays originating in the formation as a result of thermal neutron capture. The gamma rays strike the walls of the counter releasing photoelectrons which in turn cause ionization which can be detected by the counter. Although such detectors are very rugged, they suffer from the disadvantage of not directly counting the slowed down neutrons.
For a thermal or epithermal neutron detector placed at a sufficiently large distance, for example, 15 inches from the source, it can be shown that the count rate of the detector is of the form A exp(-r/L) where A is some constant which depends on the source-detector distance and the counting efficiency of the detector, r is the distance between source and detector, and L is some parameter which depends on the slowing down (of neutrons) properties of the formation, i.e., the porosity. For a formation containing no hydrogen, L will be relatively large as compared with a formation which is quite porous where L will be significantly smaller.
It is important to note that the transport of fast neutrons through a formation is characterized by three phases: (1) slowing down to thermal energy; (2) diffusion at thermal energy; and (3) capture by a formation nucleus accompanied by the emission of a characteristic gamma ray by the excited nucleus. Only the first phase gives information related directly to the Presence of hydrogen.
Since neutrons are not charged particles, their detection presents some special problems. The better detectors usually depend on the neutron undergoing some kind of nuclear reaction, one of whose products is in turn an ionizing particle such as an alpha particle. As a result of improvements in technology, the single detector neutron sonde using a heavy-walled Geiger counter was modified with the replacement of the Geiger counter by a He.sup.3 proportional counter (normal He is He.sup.4) He.sup.3 has an unusually high capture cross section for thermal neutrons, and the reaction products (ionizing) are a proton and a triton (H.sup.3). A proportional counter is used since it gives good discrimination against gamma rays.
The single He.sup.3 detector neutron sonde (detecting epithermal neutrons) was thereafter replaced by a two detector neutron sonde (detecting thermal neutrons). The two detector sonde was viewed as being less sensitive to effects of borehole conditions. Thermal detection of neutrons was chosen because count rates were higher than with epithermal detection. In this development, the ratio of the count rates of the two detectors (near and far from the source) are determined. Instead of looking at the spatial distribution of neutrons, the rate of change of the spatial distribution is being observed. A further refinement of this technique is to look at the rate of change of the spatial distribution for epithermal neutrons.
The foregoing description of prior art nuclear formation logging device relates primarily to wire line devices wherein the formation evaluation is done after drilling is completed. More recently, a new generation of formation evaluation tools has been developed which evaluate the earth formation without interrupting the drilling of a borehole. These tools are known as measurement-while-drilling or MWD tools. A typical commercial MWD tool (such as is available from Teleco Oilfield Services, Inc., assignee of the present application) may measure such downhole conditions as the so-called weight-on-bit or "WOB" as well as the torque acting on the bit, the azimuth direction and the angle of inclination of the borehole, borehole temperature, mud resistivity and various characteristics of the earth formations penetrated by the bit. The output signals of the various sensors are coupled to circuits which selectively control a downhole pressure pulse signaler in the tool for successively transmitting and/or recording encoded data signals (i.e, pressure pulses) representative of these real-time measurements through the mud stream in the drill string to suitable detecting-and-recording apparatus at the surface.
It will, of course, be appreciated that MWD tools have been proposed heretofore for providing real-time measurements of different radioactivity characteristics of earth formations being penetrated by the drill bit. Since measurement of natural gamma radiation requires only a gamma ray detector and typical circuits to control the signaler, it has not been difficult to provide MWD tools with that instrumentation. Conversely, to measure other radioactivity characteristics of earth formations, a MWD tool must also have an appropriate source of radiation (e.g., radioactive chemical source) as described above. It is far more difficult to construct a MWD tool of this type (which includes a source of radiation). While such tools have been disclosed (for example, see U.S. Pat. Nos. 4,814,609 and 4,829,176), there is a continuing need for improved MWD tools for nuclear well logging which include nuclear sources.
An improved MWD nuclear well logging tool which addresses many of the problems of the prior art is disclosed in U.S. Application Ser. No. 710,822 filed Jun. 5, 1991, which is assigned to the assignee hereof and incorporated herein by reference. The MWD tool of USSN 710,822 comprises a two detector neutron tool. In accordance with an important feature of USSN 710,822, the detectors incorporate the Li.sup.6 isotope of lithium (i.e., Li.sup.6 I crystal or Li.sup.6 doped glass). The reaction products resulting from a neutron interacting with Li.sup.6 are an alpha particle and a triton. The lithium crystal or glass is fixed to the face of a photomultiplier tube and the light scintillations which occur therein as a result of neutrons interacting with the lithium are detected and the resultant signal is amplified by the photomultiplier. The lithium crystal or glass is wrapped with a reflective material to improve the light collection for the photomultiplier tube. These detector components are all appropriately packaged for reducing vibrational damage.
Heretofore, it has been generally accepted that lithium crystal or glass detectors were not practical for tools of this type because of problems associated with gamma ray discrimination. In the case of the He.sup.3 proportional counter, the pulse heights from neutrons are usually an order of magnitude larger than those arising from gamma rays, making discrimination quite simple. For Li.sup.6 I and Li.sup.6 glass, the pulse heights from neutrons and gamma rays are comparable in magnitude. Of the two scintillators, Li.sup.6 I is inherently more sensitive to gamma rays because of the presence of iodine which is a high Z (atomic number) material. Nonetheless, the choice of Li.sup.6 glass does not remove the problem of gamma ray discrimination. However, in accordance with another important feature of USSN 710,822, gamma ray discrimination is accomplished using a novel data processing technique. Using this technique, after a spectrum of particle energies has been acquired, a microprocessor will fit a curve (e.g., an exponential curve) to the spectrum that approximates the portion of the spectrum contributed by the gamma rays. After the gamma characterization is done, the novel software then strips the gamma rays out of the raw spectrum. This is accomplished by subtracting the gamma ray spectrum from the raw spectrum. However, with large detectors a second peak is detected which is also attributed to gamma rays. This peak is the hydrogen absorption peak which is generally centered at about 2.2 MeV. Unfortunately, the gamma ray stripping technique of USSN 710,822 fails to remove this peak, thereby resulting in a neutron count which includes gamma rays attributed to hydrogen absorption. In turn, this leads to less than desired accuracy in the borehole logs derived from the neutron tool of USSN 710,822.